There are a great many instruments for measuring pressure and fundamentally many rely on pressure causing mechanical deformation of a mechanical structure which then causes an indicator to move which allows the user to observe a value of pressure. In some fields, such as medical devices or where extreme purity of gases must be maintained, it is important to measure pressure with a single element that can be made from a material compatible with the application. Conventional bellows can be used to measure pressure by visually monitoring extension of the bellows; however, bellows are difficult to manufacture with thin enough walls for measuring very low pressures, and variations in wall thickness during manufacturing can lead to significant variations in pressure sensitivity. A 10% variation in wall thickness can lead to 20% or more variation in pressure indicated depending on the design of the bellows' convolutions. One of the causes of wall thickness variation is the difficulty with which the convolutions are made; most bellows are made using blow molding which does not lead to accurate wall thickness, particularly in the corners, and error in thickness in the corners leads to variation in effective diameter. The diameter of a plate, which the convolution effectively is with regard to predicting compliance, varies with the diameter squared. Material thickness variation in the acute angle of the corners where two convolutions meet is typically much greater than the thickness variation on the plate region; hence it causes a more significant variation in the effective plate diameter and hence compliance of the bellows. This makes measurement of very small pressures, on the order of millibar to centibars, very difficult.
If the bellows can be injection molded, greater thickness control can be obtained, but then the issue is how to remove the bellows from the mold without breaking the bellows? This is done by making the convolutions not too deep so the bellows will not be stretched too much when removing it from the mold core. Once again, it becomes difficult to make thin bellows for accurate low pressure measurement.
U.S. Pat. No. 4,844,486 describes a spiral bellows to act as a boot to cover a joint. The spiral shape helps to return lubricant to the joint as it is shed by the rotating joint. The spiral helps with manufacture and provides good axial compliance. European patent EP1195545 describes a spiral bellows for a fluid conduit, where the spiral bellows form creates less noise when fluids flow through the bellows than if a conventional bellows were used. These bellows, however, have a very shallow convolution depth and are meant not for measuring pressure but to allow lateral flexibility. Hence they would be poor for measuring pressure.
U.S. Pat. No. 7,383,736 describes the use of a conventional circular cross section bellows for use as a pressure sensor, wherein the end of the bellows is close to the wall of an enshrouding chamber. The bellows is operated as an inside out pressure sensor, where the pressure on the inside of the bellows is maintained at atmospheric by connection to the outside world, the pressure on the outside of the bellows is that of the pressure source. Hence with increasing pressure, the bellows compresses. No structure is perfect, and hence pressure will cause some radial motion and thus contact between the end of the bellows and the outer wall. For larger pressures the bellows will tip and make contact with the wall of the enshrouding structure, and the resultant drag force will create an error in pressure measurement. The patent shows an increase in the bellows end diameter or a connection between the bellow's end and a piston which are both attempts to keep the bellows centered, but no matter what, if there is mechanical contact, and upon application of pressure there will be, there will be a parasitic friction force that causes error in pressure measurement. Furthermore, in this compression design, the wall thickness of the bellows cannot be too thin or in compression the bellows will buckle; hence it is inherently limited in how small a pressure it can sense while providing reasonable amount of axial compression so as to make it easy to discern visually by the user.
U.S. Pat. No. 3,911,904 is a sphygmomanometer where instead of a column of mercury, a balloon constrained on its sides by walls of a chamber is allowed to expand axially to indicate pressure. The only problem is that the device simply will not work because the balloon will not expand in a near fashion with pressure because the physical contact of the balloon with the chamber walls will constrain it and prevent it from extending axially.
U.S. Pat. No. 4,501,273 is a device that connects to a syringe that forms a cavity where the extension of a bladder within the cavity is controlled by constant force springs, such that the device maintains constant pressure in a endotracheal cuff. The device does not measure pressure, nor can it, but it acts as a relief valve/capacitor. This addresses the issue of controlling endotracheal cuff pressure in a very different way, and does not enable the doctor to change the pressure in accordance with what the patient or procedure may require.
U.S. Pat. No. 5,103,670 is from the family of pressure gage instruments where the function of the bellows is to contain the air pressure and transfer the forces to an indicator (rotary) mechanism, but as such the mechanism has considerable sliding friction and hence there will be substantial errors in measurement for low pressures. As this device is indicated for measuring tire pressure, the error sin measurement could be expected to be tolerable for that application. U.S. Pat. No. 4,114,458 functions in a similar manner and has similar fatal drawbacks with regard to precision pressure measurement. U.S. Pat. No. 4,966,035 replaces the bellows with a spring, piston and seal and thus has even greater parasitic errors.
U.S. Pat. No. 5,336,183 shows a syringe with a secondary pressure measurement chamber in parallel where pressure is sensed by a piston constrained radially by seals with the enclosing chamber, and axially by a spring. Hence any pressure forces generated will first have to overcome the static friction sealing forces and a large error in measurement will occur.
U.S. Pat. No. 5,722,955 shows a syringe with its plunger's distal end affixed with a pressure sensitive open cell foam material coated with an impermeable membrane; however, this means that the compression will be proportional to pressure. Hence it might only present a reasonable visual displacement for a substantial fraction of one atmosphere of pressure. Furthermore, it will not provide the same reading in Denver as in Los Angeles or anyplace where the barometric pressure is different than from where it was manufactured.
U.S. Pat. No. 6,485,471 shows a plunger used to push on a bellows, thus expelling the contents. Since the bellows elasticity is in series with the plunger and the fluid line leading from it, there is no way to ascertain what portion of the force on the plunger is due to the pressure in the line and that due to the elasticity of the bellows. In addition, lateral motion of the plunger would create contact with the enshrouding cylinder walls creating a parasitic force further reducing accuracy. U.S. Pat. No. 6,042,092 shows a similar device where the bellows is intended to provide a spring return force to a plunger, this time with a toggle linkage also present. Once again, there can be no true accurate indication of pressure. U.S. Pat. No. 7,018,359, U.S. Pat. No. 5,439,178 and U.S. Pat. No. 5,935,084 are also of similar design with regard to the placement And action of forces and thus have similar shortcomings that prevent it being able to provide accurate pressure readings.
Furthermore, there is a fundamental problem associated with using an axially compliant cantilevered member to measure pressure: axial compliance equates to a low effective modulus of elasticity, which in turn means that gravity will cause any such structure to sag considerably under its own weight; although the modulus of elasticity of the rubber in a bellows may be several MPa, the convolutions make it effectively to be orders of magnitude lower so that any cantilever beam loaded under its own weight but having such a low modulus will droop substantially. Once the drooping structure makes contact with the walls of a protective enclosure, a parasitic frictional force will be encountered which can cause significant pressure measurement errors if very fine pressure measurements attempted.
In addition, a bellows-type pressure sensor must start with a large volume to provide structure that can then elastically deform. This large volume is undesirable if a fluid is the working media because the fluid must then be disposed of if the application is single use. For many medical applications, this adds to the mass of material that has to be treated as bio-waste.
What is desired is a movable structure whose motion is proportional to pressure applied. In the case of the application to a pressure measuring syringe, the motion of the syringe plunger ideally would cause the motion of the movable structure as pressure is increased. The movable structure could be attached to the syringe plunger or to a secondary structure attached to the syringe or line coming from the syringe. When thinking of a structure that moves when pressure is applied, one thinks of a piston, yet static friction must be eliminated. To one skilled in the art of pistons and seals, motion without friction can be accomplished by a rolling diaphragm seal; however, these seals are noted for having a very low constant force resistance, hardly suitable for pressure measurement.
These limitations are overcome in the present invention with the use of a resilient outer tube and an inner resilient closed-end tube connected by a resilient rolling diaphragm, where at least one of the outer or inner tubes or the wall thickness tapers. By tapering either or both of the tubes and/or/nor the thickness the sensitivity and linearity of the device to pressure can be controlled.